1. Field of the Invention
The present invention relates to correction of a shift in the projected image of a pattern formed on a substrate of a lithographic projection apparatus caused by variations in the position of a pattern surface of a mask along the optical axis of the apparatus.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from United States Patents U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement and/or inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical, mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-0672504.
In order to increase the degree of integration of semiconductor devices and keep pace with Moore's law, it will be necessary to provide lithographic projection apparatus capable of printing practical minimum line widths of 25-100 nm. Currently available photolithographic tools using 193 nm and 157 nm radiation can produce pattern features having a resolution (in nm) according to the well known equation R=k1·λ/NA, where R is the resolution, k1 is a constant that is dependent on the radiation sensitive material (resist) used, λ is the wavelength of the radiation, and NA is the numerical aperture. A lower limit; of k1, is 0.25 and lithographic projection apparatus having a NA of 0.85 are currently available. Difficulties in optical design make increasing the NA difficult. As k1 and NA A are generally considered to be at their limits, the ability to decrease the resolution, i.e, print smaller pattern features, of lithographic projection apparatus appears to be dependent on, decreasing the wavelength λ of the radiation.
Resolution enhancement techniques, such as phase shifting masks, optical proximity correction, sub-resolution assist features, and off-axis illumination, have allowed lithographic projection apparatus using 193 and 157 nm radiation to print pattern features of 100 nm resolution. In order to print pattern features smaller than 100 nm, there are currently being developed lithographic projection apparatus using radiation in the soft X-ray region having a wavelength of 5 to 15 nm, often-referred to as extreme ultraviolet (EUV).
The use of EUV radiation in a lithographic projection apparatus present several problems. EUV radiation is absorbed by all materials, including air. The EUV radiation source, illumination system, the projection system, the mask (reticle) and mask table, and the substrate (wafer) and the substrate table must be placed in a vacuum to prevent absorption of the EUV projection beam. Reflective masks are used in EUV lithographic projection apparatus as no materials exist for forming a mask which can efficiently transmit EUV radiation without absorption. It is also difficult to prepare a beam splitter for EUY radiation. It is therefore necessary that the EUV projection beam be radiated obliquely with respect to the mask to allow the reflected beam to reach the projection system without being blocked by illumination system optics.
Because the beam of radiation is radiated obliquely with respect to the mask, the patterned side of the mask is non-telecentric. The displacement of the mask along the optical (Z) axis results in a change in the magnification and a displacement of the exposed area in the scanning (Y) direction that results in a change of the position of the image on the wafer. There are several causes of variations of the mask pattern in the Z direction.
Mask unflatness is one cause of variations in the Z direction. Referring to FIG. 2, an unflat mask MA1 reflects a beam of radiation PB incident on the mass MA at an angle α (e.g. 50 mrad) at a point B. The beam PB is reflected by the mask MA1 onto a wafer W coated with a radiation sensitive material (photoresist) RSM. For the sake of simplicity, the projection system is omitted. Point B is displaced a distance ΔYw=+/−tan α·ΔZ from a point A that would be imaged if the mask were truly flat the mask unflatness results in a shift in the mask pattern image on the wafer by an amount ΔYw=+/−tan α·ΔZ/M, where M is the magnification of the projection system (not shown) and the sign of the amount ΔYw depends on the image reversal characteristic of the projection system. The shift ΔYw in the pattern image on the wafer results in an overlay (superimposing) error in the semiconductor manufacturing process. In a semiconductor device having a critical dimension of 100 nm, the maximum overlay error is not more than 30 nm. Other causes of overlay error besides variations in the Z direction include positioning/alignment accuracy between the mask and the wafer, the positioning accuracy of the wafer stage, including the stepping accuracy, and the distortion of the projection system, which may cause overlay errors of approximately 10 nm.
Another cause of variations in the Z direction in an EUV lithographic tool is the necessity of mounting of the mask on its back surface opposite the patterned surface. As the mask must be contained in a vacuum it must be clamped on its back surface, for example by an electrostatic chuck. In lithographic tools in which the use of a vacuum is not necessary, the patterned and mounting sides of the mask are the same. The mask focal plane is thus established at the plane of the mask stage platen. Accordingly, knowledge of the mask stage position in all six degrees of freedom results in knowledge of the mask patterned surface in all six degrees of freedom. Clamping of the mask on its back surface, as required in an EUV lithographic tool, causes the mask focal plane position relative to the mask stage position to be a function of mask flatness, mask thickness and mask thickness variation. In addition, framing blades are used as a field diaphragm at the mask focal plane and make determination of the mask focal plane difficult with current out of plane gauges.
Referring to FIG. 3, a mask MA2 displaced in the Z direction (as shown in dashed lines) by an amount ΔZ results in a shift of the image of point C to point D by an amount ΔYMA=tan α·ΔZ. Unflatness of the mask MA2 and rotation of the mask MA2 about the X and Y axes also cause variations in the Z direction and shifting of the pattern image in the Y (scanning) direction. Displacement of the mask MA2 in the Z direction results in a shift in the mask pattern image on the wafer by an amount ΔYw=+/−tan αΔZ/M, where M is the magnification of the projection system (not shown) and the sign of the amount ΔYw depends on the image reversal characteristic of the projection system.
For the sac of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of the design types for directing, shaping or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple sEage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and U.S. Pat. No. 6,262,796.